Switching power converters offer higher efficiency as compared to linear regulators. Although linear regulators are relatively inexpensive, they regulate a lower output voltage from a higher input voltage by simply burning the difference as heat. As a result, a linear regulator typically burns more power than is actually supplied to the load. In contrast, a switching power converter regulates its output voltage by delivering relatively small increments of energy through the cycling of a power switch. The power switch in a switching power converter is either off or on so that efficiency is markedly improved as compared to linear regulators. Switching power converters such as a flyback converter are thus typically used to charge the batteries for mobile devices.
Not only are flyback converters efficient, their transformers provide a safe isolation of the device being charged from the AC mains. But this isolation leads to a regulation problem since the output voltage (or output current) is delivered from the secondary winding of the transformer whereas the power switch is connected to the transformer's primary winding. A natural location for a controller is thus on the primary side to regulate the output power delivery by regulating the cycling of the power switch. But a primary-side controller cannot simply read the output voltage directly such as through a wire or lead because the isolation between the primary and secondary sides of the transformer would then be destroyed. It is thus conventional for the primary-side controller to receive feedback information about the output voltage through an isolating communication channel such as an optoisolator. Although the primary-side controller can then receive feedback information, the optoisolator causes stability issues and adds expense to the flyback converter design.
To avoid such stability and expense, flyback converter design evolved to use primary-only feedback. In primary-only feedback, the primary side senses the voltage on an auxiliary winding (or on the primary winding) at the moment when the secondary current has ramped down to zero following a cycling of the power switch. This moment is referred to as the transformer reset time and represents an ideal time to sample the output voltage indirectly through the auxiliary winding voltage since there is a linear relationship between the output voltage and the auxiliary winding voltage at the transformer reset time. But following the transformer reset time, the auxiliary winding voltage will begin resonantly oscillating. It is thus difficult to sample the auxiliary winding voltage at the precise moment of the transformer reset time. Moreover, noise and other non-idealities limit the accuracy of primary-only feedback. For example, the accuracy of regulating the output voltage through primary-only feedback is limited by non-perfect coupling of the transformer, secondary-side rectifier diode forward voltage drop variation, and synchronous rectification MOSFET Rds variation, component variation, and the converter load range and operation conditions.
Given the limitations for primary-only feedback, it is inapplicable to applications that demand extreme accuracy such as voltage regulation with no more than one per cent error. To provide additional accuracy, various secondary-side regulation schemes have been implemented. As implied by the name, secondary-side regulation involves sensing the output voltage and comparing the sensed output voltage to a reference voltage to develop an error voltage. The error voltage is filtered to form a control voltage that is further processed by a power switch controller to control the power switch cycling. In one form of conventional secondary-side regulation, the secondary-side controller does the processing of the control voltage to generate an activity signal that forces the power switch to cycle on. In such an embodiment, the primary side may include a rudimentary controller that then switches the power switch off after a fixed peak current or on time is reached. Alternatively, the secondary-side controller may also control the off times as well as the on times for the power switch.
In lieu of sending an activity signal to stimulate a power switch cycle, other embodiments for conventional secondary-side regulation send the control voltage itself to a primary-side controller. The primary-side controller may then process the control voltage to control the power switch cycling accordingly. But regardless of whether an activity signal or the control signal is transmitted from the secondary side to the primary side, certain modes of operation are then hampered by the transition from primary-side feedback to secondary-side regulation. For example, the charging of a lithium battery must follow a certain transition between constant-voltage and constant-current modes of operation. In particular, modern smartphones are typically rather expensive yet their lithium battery (or batteries) is integrated permanently or semi-permanently in the smartphone's housing. Should a flyback converter destroy the battery through an improper charging sequence, the entire smartphone is then destroyed. It is thus critical that the proper constant-voltage and constant-current modes of operation be properly regulated during the battery charging process. But a secondary-side controller requires the insertion of a current sense resistor on the secondary side (or some other means of sensing the output current) to regulate a constant-current power delivery to the load. The addition of a current sense resistor into the output current path on the secondary side of a flyback converter with conventional secondary-side regulation lowers efficiency and increases manufacturing complexity and cost.
Accordingly, there is a need in the art for improved forms of secondary-side regulation.